Hi,
To use certificate logins you need to do two things:
1. Enable certificate login on the server-side, refer to http://docs.sun.com/app/docs/doc/819-2650/6n4u4dtu8?a=view "To Set Up Certificate-Based Login"
2. Import the user certificate which matches that stored in your corporate directory into the email client (refer to client manuals/instructions for that step).
Getting certificate logins working isn't a simple process unfortunately as you need a fair number of steps to go right. Also the advantages are not that great as the user has to unlock the certificate database anyway, so you aren't really cutting down on logins.
At my previous work we ended up canning certificate logins as they proved more of a nuisance then anything.
Shane.